I. Field of the Invention
The present invention relates generally to object position determination using satellites. More specifically, the present invention relates to a method for determining the position of a user terminal in a satellite communications system using measurements performed at both ends of a communication link.
II. Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
A variety of multiple access communications systems have been developed for transferring information among a large number of system users. These techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) spread-spectrum techniques, the basics of which are well known in the art. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters," and U.S. patent application Ser. No. 08/368,570, filed Jan. 4, 1995, entitled "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System, For Tracking Individual Recipient Phase Time And Energy," U.S. Pat. No. 5,691,974, which are both assigned to the assignee of the present invention, and are incorporated herein by reference.
The above-mentioned patent documents disclose multiple access communications systems in which a large number of generally mobile or remote system users employ user terminals to communicate with other system users or users of other connected systems, such as a public telephone switching network. The user terminals communicate through gateways and satellites using CDMA spread-spectrum type communications signals.
Communications satellites form beams which illuminate a "spot" or area produced by projecting satellite communications signals onto the Earth's surface. A typical satellite beam pattern for a spot comprises a number of beams arranged in a predetermined coverage pattern. Typically, each beam comprises a number of so-called sub-beams (also referred to as CDMA channels) covering a common geographic area, each occupying a different frequency band.
In a typical spread-spectrum communications system, a set of preselected pseudorandom noise (PN) code sequences is used to modulate (i.e., "spread") information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communications signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In a forward communications link (that is, in a communications link originating at a gateway and terminating at a user terminal), PN spreading codes or binary sequences are used to discriminate between signals transmitted by a gateway over different beams, and to discriminate between multipath signals. These PN codes are typically shared by all communications signals within a given sub-beam.
In a typical CDMA spread-spectrum system, channelizing codes are used to discriminate between signals intended for particular user terminals transmitted within a satellite beam on the forward link. That is, a unique orthogonal channel is provided for each user terminal on the forward link by using a unique "channelizing" orthogonal code. Walsh functions are generally used to implement the channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems.
Typical CDMA spread-spectrum communications systems, such as disclosed in U.S. Pat. No. 4,901,307, contemplate the use of coherent modulation and demodulation for forward link user terminal communications. In communications systems using this approach, a "pilot" carrier signal (hereinafter referred to as a "pilot signal") is used as a coherent phase reference for forward links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a gateway throughout a region of coverage. A single pilot signal is typically transmitted by each gateway for each beam used for each frequency used. These pilot signals are shared by all user terminals receiving signals from the gateway.
Pilot signals are used by user terminals to obtain initial system synchronization and time, frequency, and phase tracking of other signals transmitted by the gateway. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of other system signals or traffic signals. This technique allows many traffic signals to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism.
When a user terminal is not involved in a communications session (that is, the user terminal is not receiving or transmitting traffic signals), the gateway can convey information to that particular user terminal using a signal known as a paging signal. For example, when a call has been placed to a particular mobile phone, the gateway alerts the mobile phone by means of a paging signal. Paging signals are also used to distribute traffic channel assignments, access channel assignments, and system overhead information.
A user terminal can respond to a paging signal by sending an access signal or access probe over the reverse link (that is, the communications link originating at the user terminal and terminating at the gateway). The access signal is also used when a user terminal originates a call.
When communications are required with a user terminal, the communications system may need to determine the position of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. One aspect of this consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory, and handles all calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries or contracted services.
A crucial requirement in position determination for a satellite-based communications system is speed. When communications are required with a particular user terminal, the gateway that will serve the user terminal should be selected rapidly. For example, a mobile telephone user is not likely to tolerate a delay of more than a few seconds when placing a call. The need for positioning accuracy in this situation is less important than the need for speed. An error of less than 10 kilometers (km) is considered adequate. In contrast, most conventional approaches to satellite-based position determination emphasize accuracy over speed.
One conventional approach is that employed by the U.S. Navy's TRANSIT system. In that system, the user terminal performs continuous Doppler measurements of a signal broadcast by a low-Earth orbit (LEO) satellite. The measurements continue for several minutes. The system usually requires two passes of the satellite, necessitating a wait of more than 100 minutes. In addition, because the position calculations are performed by the user terminal, the satellite must broadcast information regarding its position (also known as "ephemeris"). Although the TRANSIT system is capable of high accuracy (on the order of one meter), the delay involved is unacceptable for use in a commercial satellite communications system.
Another conventional approach is that employed by the ARGOS and SARSAT (Search and Rescue Satellite) systems. In that approach, the user terminal transmits an intermittent beacon signal to a receiver on the satellite, which makes frequency measurements of the signal. If the satellite receives more than four beacon signals from the user terminal, it can usually solve for the user terminal's position. Because the beacon signal is intermittent, an extended Doppler measurement, such as that performed by the TRANSIT system, is unavailable- Further, the solution is ambiguous, providing a possible position on each side of the satellite sub-track (that is, a line on the Earth's surface that is directly below the satellite's path).
Another conventional approach is that employed by the Global Positioning System (GPS). In that approach, each satellite broadcasts a time-stamped signal that includes the satellite's ephemeris. When the user terminal receives a GPS signal, the user terminal measures the transmission delay relative to its own clock and determines a pseudo-range to the transmitting satellite's position. The GPS system requires three satellites for two-dimensional positioning, and a fourth satellite for three-dimensional positioning.
One disadvantage of the GPS approach is that at least three satellites are required for position determination. Another disadvantage of the GPS approach is that, because the calculations are performed by the user terminal, the GPS satellites must broadcast their ephemeris information, and the user terminal must possess the computational resources to perform the required calculations.
One disadvantage of all of the above-described approaches is that the user terminal must have a separate transmitter or receiver, in addition to that required to process communication signals, in order to use those approaches.
Another conventional approach is that disclosed in commonly-owned U.S. Pat. No. 5,126,748, which issued Jun. 30, 1992, entitled "Dual Satellite Navigation System And Method," which is incorporated herein by reference. This approach employs two satellites to actively determine the user terminal's position through trilateration. One disadvantage with this method is that the solution is ambiguous, providing two possible positions. Further information is needed to resolve the ambiguity.
What is needed, therefore, is a satellite-based position determination system capable of rapid, unambiguous position determination.